Cellulitis

The most common causes of cellulitis are

Cellulitis is most often caused by group A beta-hemolytic streptococci (eg, S. pyogenes) or S. aureus. The skin barrier is usually compromised.

Streptococci cause diffuse, rapidly spreading infection because enzymes produced by the organism (streptokinase, DNase, hyaluronidase) break down cellular components that would otherwise contain and localize the inflammation.

Staphylococcal cellulitis is typically more localized and usually occurs in open wounds or cutaneous abscesses Cutaneous Abscess A cutaneous abscess is a localized collection of pus in the skin and may occur on any skin surface. Symptoms and signs are pain and a tender and firm or fluctuant swelling. Diagnosis is usually... read more .

Methicillin-resistant S. aureus (MRSA-USA300) is the predominant community strain of MRSA in the United States (community-associated MRSA [CA-MRSA]) (1 Etiology reference Cellulitis is acute bacterial infection of the skin and subcutaneous tissue most often caused by streptococci or staphylococci. Symptoms and signs are pain, warmth, rapidly spreading erythema... read more ). If S. aureus is suspected, MRSA infection should now be considered the most probable etiology. Patients who are exposed to MRSA in a hospital or nursing facility may have a MRSA strain that has a different pattern of resistance from that of MRSA-USA300.

Less common causes of cellulitis are

Animal bites Human and Mammal Bites Human and other mammal bites (mostly dog and cat bites, but also squirrel, gerbil, rabbit, guinea pig, and monkey bites) are common and occasionally cause significant morbidity and disability... read more may result in cellulitis and are often polymicrobial; Pasteurella multocida is often the cause in cat bites, and Pasteurella or Capnocytophaga species are typically responsible in dog bites.

Immersion injuries in fresh water may result in cellulitis caused by Aeromonas hydrophila. Immersion injuries in warm salt water may result in cellulitis caused by Vibrio vulnificus Noncholera Vibrio Infections Noncholera vibrios include the gram-negative bacteria Vibrio parahaemolyticus, V. mimicus, V. alginolyticus, V. hollisae, and V. vulnificus; they may cause... read more .

Patients who are immunocompromised may become infected by opportunistic organisms, including gram-negative bacteria (such as Proteus Proteeae Infections The Proteeae are normal fecal flora that often cause infection in patients whose normal flora have been disturbed by antibiotic therapy. The Proteeae constitute at least 3 genera of gram-negative... read more , Serratia Klebsiella, Enterobacter, and Serratia Infections The gram-negative bacteria Klebsiella, Enterobacter, and Serratia are closely related normal intestinal flora that rarely cause disease in normal hosts. Diagnosis is by... read more , Enterobacter Klebsiella, Enterobacter, and Serratia Infections The gram-negative bacteria Klebsiella, Enterobacter, and Serratia are closely related normal intestinal flora that rarely cause disease in normal hosts. Diagnosis is by... read more , or Citrobacter), anaerobic bacteria Overview of Anaerobic Bacteria Bacteria can be classified by their need and tolerance for oxygen: Facultative: Grow aerobically or anaerobically in the presence or absence of oxygen Microaerophilic: Require a low oxygen concentration... read more , and Helicobacter Helicobacter pylori Infection Helicobacter pylori is a common gastric pathogen that causes gastritis, peptic ulcer disease, gastric adenocarcinoma, and low-grade gastric lymphoma. Infection may be asymptomatic or... read more and Fusarium Miscellaneous Opportunistic Fungi Many yeasts and molds can cause opportunistic, even life-threatening infections in patients who are immunocompromised. These infections only rarely affect immunocompetent people. Yeasts tend... read more species. Mycobacteria Nontuberculous Mycobacterial Infections There are over 170 recognized species of mycobacteria, mostly environmental. Environmental exposure to many of these organisms is common, but most exposures do not cause infection and many infections... read more may rarely cause cellulitis.

Risk factors include skin abnormalities (eg, trauma, ulceration, fungal infection, other skin barrier compromise due to preexisting skin disease), which are common among patients with chronic venous insufficiency Chronic Venous Insufficiency and Postphlebitic Syndrome Chronic venous insufficiency is impaired venous return, sometimes causing lower extremity discomfort, edema, and skin changes. Postphlebitic (postthrombotic) syndrome is symptomatic chronic... read more or lymphedema Lymphedema Lymphedema is edema of a limb due to lymphatic hypoplasia (primary) or to obstruction or disruption (secondary) of lymphatic vessels. Symptoms and signs are brawny, fibrous, nonpitting edema... read more . Scars from saphenous vein removal for cardiac or vascular surgery are common sites for recurrent cellulitis, especially if tinea pedis is present. Frequently, no predisposing condition or site of entry is evident.

Infection is most common in the lower extremities. Cellulitis is typically unilateral; stasis dermatitis Stasis Dermatitis Stasis dermatitis is inflammation, typically of the skin of the lower legs, caused by chronic edema. Symptoms are itching, scaling, and hyperpigmentation. Ulceration can be a complication. Diagnosis... read more closely mimics cellulitis but is usually bilateral.

The major findings are local erythema and tenderness and, in more severe infections, often lymphangitis and regional lymphadenopathy. The skin is warm, erythematous, and edematous, often with surface appearance resembling the skin of an orange (peau d’orange). The borders are usually indistinct, except in erysipelas Erysipelas Erysipelas is a type of superficial cellulitis with dermal lymphatic involvement. Diagnosis is clinical. Treatment is with oral or parenteral antibiotics. (See also Overview of Bacterial Skin... read more (a type of cellulitis with sharply demarcated margins). Petechiae are common; large areas of ecchymosis are rare.

Vesicles and bullae may develop and rupture, occasionally with necrosis of the involved skin.

Cellulitis may mimic deep venous thrombosis but can often be differentiated by one or more features (see table ).

Most cellulitis is nonpurulent. However, cellulitis sometimes is accompanied by one or more pustules, furuncles Furuncles and Carbuncles Furuncles (boils) are skin abscesses caused by staphylococcal infection, which involve a hair follicle and surrounding tissue. Carbuncles are clusters of furuncles connected subcutaneously,... read more , or abscesses Cutaneous Abscess A cutaneous abscess is a localized collection of pus in the skin and may occur on any skin surface. Symptoms and signs are pain and a tender and firm or fluctuant swelling. Diagnosis is usually... read more with or without purulent drainage or exudate and is referred to as purulent.

Fever, chills, tachycardia, headache, hypotension, and delirium (usually indicating severe infection) may precede cutaneous findings by several hours, but many patients do not appear ill. Leukocytosis is common. Cellulitis with rapid spread of infection, rapidly increasing pain, hypotension, delirium, or skin sloughing, particularly with bullae and fevers, suggests life-threatening infection.

Diagnosis of cellulitis is by examination. Contact dermatitis Contact Dermatitis Contact dermatitis is inflammation of the skin caused by direct contact with irritants (irritant contact dermatitis) or allergens (allergic contact dermatitis). Symptoms include pruritus and... read more and stasis dermatitis Stasis Dermatitis Stasis dermatitis is inflammation, typically of the skin of the lower legs, caused by chronic edema. Symptoms are itching, scaling, and hyperpigmentation. Ulceration can be a complication. Diagnosis... read more are often misdiagnosed as cellulitis, thus leading to unnecessary antibiotic use. Contact dermatitis can often be differentiated by the presence of itching, limitation of lesions to the site of contact, absence of systemic signs, and sometimes unilateral location. Stasis dermatitis can sometimes be differentiated by features of dermatitis itself (eg, scaling, eczematous findings, lichenification), evidence of venous stasis, and bilateral location. Other disorders to consider include cutaneous T-cell lymphoma Cutaneous T-cell Lymphomas (CTCL) Mycosis fungoides and Sézary syndrome are uncommon chronic T-cell non-Hodgkin lymphomas primarily affecting the skin and occasionally the lymph nodes. (See also Overview of Lymphoma and Non-Hodgkin... read more , nummular dermatitis Nummular Dermatitis Nummular dermatitis is inflammation of the skin characterized by coin-shaped or discoid eczematous lesions. Diagnosis is clinical. Treatment may include topical corticosteroids and phototherapy... read more , and tinea infection Tinea Corporis (Body Ringworm) Tinea corporis is a dermatophyte infection of the face, trunk, and extremities. Diagnosis is by clinical appearance and by examination of skin scrapings on potassium hydroxide wet mount. Treatment... read more .

Blood cultures are useful to detect or rule out bacteremia Bacteremia Bacteremia is the presence of bacteria in the bloodstream. It can occur spontaneously, during certain tissue infections, with use of indwelling genitourinary or IV catheters, or after dental... read more in patients who are immunocompromised and in patients who have signs of systemic infection (eg, fever and leukocytosis).

Culture of involved tissue may be required in patients who are immunocompromised if they are not responding to empiric therapy or if blood cultures do not isolate an organism as well as in patients with cellulitis at the site of certain injuries (eg, animal bite wounds, penetrating injuries).

Skin and wound cultures (when wounds are present) are generally not indicated in cellulitis because they rarely identify the infecting organism.

Abscess Cutaneous Abscess A cutaneous abscess is a localized collection of pus in the skin and may occur on any skin surface. Symptoms and signs are pain and a tender and firm or fluctuant swelling. Diagnosis is usually... read more should be ruled out based on clinical findings, although bedside ultrasonography may be useful.

Antibiotics are the treatment of choice, and selection is based on the presence or absence of purulence and other risk factors for serious and/or resistant infection (1 Treatment reference Cellulitis is acute bacterial infection of the skin and subcutaneous tissue most often caused by streptococci or staphylococci. Symptoms and signs are pain, warmth, rapidly spreading erythema... read more ). Treatments are usually not given for a fixed interval but are continued until there is satisfactory clinical response—but typically for not less than 1 week.

Immobilization and elevation of the affected area help reduce edema; cool, wet dressings relieve local discomfort.

Compression therapy can help prevent repeat episodes of leg cellulitis in patients with recurrent cellulitis who have chronic lower extremity edema.

Most cellulitis resolves quickly with antibiotic therapy. Local abscesses occasionally form, requiring incision and drainage. Serious but rare complications include severe necrotizing subcutaneous infection Necrotizing Soft-Tissue Infection Necrotizing soft-tissue infection is typically caused by a mixture of aerobic and anaerobic organisms that cause necrosis of subcutaneous tissue, usually including the fascia. This infection... read more and bacteremia Bacteremia Bacteremia is the presence of bacteria in the bloodstream. It can occur spontaneously, during certain tissue infections, with use of indwelling genitourinary or IV catheters, or after dental... read more with metastatic foci of infection.

Recurrences in the same area are common, sometimes causing serious damage to the lymphatics, chronic lymphatic obstruction, and lymphedema.